Heretofore, titanium dioxide has been widely used as a white pigment, because of its excellent hiding power and superior tinting strength. When titanium dioxide is processed into ultrafine particles having a fine particle size, it is known to exhibit new optical properties different from those of conventional titanium dioxide pigments. On the basis of this knowledge, in recent years, extensive studies have been carried out on titanium dioxide fine particles. Regarding such optical properties, for example, when titanium dioxide is formed into fine particles having a diameter approximately half the wavelength of visible light, the scattering effect of the titanium dioxide fine particles is reduced considerably, and as a result, the fine particles transmit visible light while exerting excellent ultraviolet absorbing effect. Therefore, titanium dioxide fine particles have become of interest as an ultraviolet absorber capable of transmitting visible light.
Meanwhile, when titanium dioxide particles are formed into fine particles, they increase in the surface area, and the effect of the surface of the particles surpasses the effect of the bulk of the particles. As a result, the photocatalytic effect of the titanium dioxide particles comes to stand out. The photocatalytic mechanism of titanium dioxide is considered to proceed on the basis of the following mechanism. First, when titanium dioxide fine particles are irradiated with light, electrons and holes generated inside the fine particles react with water and oxygen near the surface of the fine particles, to thereby produce hydroxyl radicals and hydrogen peroxide. By the strong oxidation-reduction action of the hydroxyl radicals and hydrogen peroxide, harmful organic substances are purified into carbon dioxide gas and water. Such a photocatalytic action of titanium dioxide fine particles is said to continue semi-permanently insofar as titanium dioxide fine particles, light, water, and oxygen are present. Studies have been carried out on environmental purification utilizing such photo-functional properties of titanium dioxide ultrafine particles, such as antibiosis, deodorization, antifouling, air cleaning, and water cleaning.
Attempts to utilize such a photo-functional activity of titanium dioxide fine particles have led to proposing a method of kneading titanium dioxide fine particles into an easily handled medium such as a fibrous material or a plastic formed article, and a method of applying the fine particles to the surface of a substrate such as cloth or paper. In addition, a method of applying the fine particles to the surface of an object so as to cause the surface to exhibit excellent hydrophilicity has been proposed. However, not only harmful organic substances or environmental contaminants, but also media such as fiber, plastic, and paper are easily decomposed and impaired by the strong photocatalytic activity of titanium dioxide, and, in view of durability, this stands as an obstacle to practical use. Also, because titanium dioxide fine particles are easily handled, a coating material prepared by mixing titanium dioxide fine particles and a binder has been developed. However, a binder exhibiting the durability sufficiently high to overcome the problem (obstacle) of activity on the media has not yet been found. Furthermore, in order to impart to the coating material excellent hydrophilicity—attributed to the photo-functional property of titanium dioxide—the coating material must contain silica particles, silicone molecules, etc. in addition to titanium dioxide fine particles, and the coating material must be subjected to photo-excitation.
With regard to the related technology, JP-A-9-225319 (the term “JP-A” as used herein means “unexamined published Japanese Patent Application”) and JP-A-9-239277 disclose a technique of preventing and suppressing the deterioration of a resin medium or a binder that would otherwise occur because of the strong photocatalytic activity of titanium dioxide particles, where there is proposed, as means therefor, a method of bearing a photoinactive compound such as aluminum, silicon, or zirconium on the surface of titanium dioxide particles such that the compound assumes a form of archipelago with steric hindrance to thereby suppress the photocatalytic activity. However, this method is disadvantageous in that, since the photoinactive compound is supported so as to form an archipelago form, a portion subjected to the strong photocatalytic activity of titanium dioxide is present at a specific site of the resin medium or binder.
JP-A-10-244166 (U.S. Pat. No. 6,180,548) proposes photocatalytic titanium dioxide prepared by coating the surface of titanium dioxide with porous calcium phosphate. However, in this case, as has been pointed out, the photocatalytic performance decreases because of the coating film; that is, the calcium phosphate layer.
International Patent Publication WO99/33566 discloses a titanium dioxide fine particle powder where a porous calcium phosphate coating layer is formed at least on a portion of the surface of titanium dioxide fine particles, and an anionic surfactant is present in the interface between the layer and the surface of the fine particles.
With respect to a slurry containing titanium dioxide having photocatalytic activity, JP-A-11-335121 discloses an anatase-type-titanium-oxide-containing slurry obtained by subjecting a titania sol solution, a titania gel, or a titania sol·gel mixture to heat treatment and pressure treatment in a closed container, and then to dispersion using ultrasonic waves or stirring.
JP-A-11-343426 discloses a photocatalytic coating material having excellent dispersion stability, which is a photocatalytic coating material containing titanium oxide and silica sol in a solvent, where the titanium oxide has a Raman spectrum peak in the range from 146 to 150 cm−1, and 95 mass % or more of the titanium oxide is occupied by anatase-type titanium dioxide.
Japanese Patent No. 2756474 discloses a substrate including a photocatalytic film formed from a photocatalytic semiconductor material and silica or silicone, wherein the surface of the film exhibits hydrophilicity through photo-excitation; and a composition used for forming the substrate.
As described above, although several techniques have been disclosed, there cannot be provided, in a industrially useful manner, a photo-functional material (powder, slurry, etc.) exhibiting photocatalytic activity and hydrophilicity which satisfies both durability and dispersion stability when used in combination with an organic material.